Biostratigraphy and palaeogeographic affinities of Early Silurian chitinozoans from central Saudi Arabia

Palynological investigations carried out on subsurface material have demonstrated the occurrence of rich and well diversified Llandovery chitinozoan assemblages in the Qusaiba and Sharawra members of the Qalibah Formation in Saudi Arabia. Both core and cuttings samples from wells located in the central part of Saudi Arabia have been investigated. The biozonation and stratigraphic correlations proposed here are exclusively based on core material. Eight local chitinozoan biozones are distinguished within the investigated Llandovery subsurface strata. Because of significant contamination due to caving in the cuttings samples these are only used to control the extension of the biozones in the intervals devoid of core samples. Several species of the most abundantly recorded chitinozoans in the Early Silurian of Saudi Arabia are typical components of contemporaneous chitinozoan assemblages from the subsurface of Libya, Algeria and to a lesser extent from outcrops in northwestern Spain (e.g. Plectochitina paraguayensis, Conochitina edjelensis, C. alargada, C. elongata, Pterochitina deichai, Belonechitina paravitrea). In addition to these typical North Gondwanan species, a few fairly ubiquitous forms are helpful for inter-palaeocontinental correlations (e.g. Spinachitina fragilis, S. maennili). The relationship existing between North Gondwanan chitinozoan assemblages and the Baltic ones are quantified for Aeronian time by calculating their respective coefficient of similarity. A great similarity exists between North Gondwana and Saudi Arabia chitinozoan populations from the Middle Llandovery. On the other hand, moderate to fairly low similarities are noticed with the contemporaneous chitinozoan populations from Baltica. Such results are consistent with the existence of a wide mid-European Rheic Ocean implying a remote latitudinal position between Northern Gondwana and Baltica and consequently contrasting climatic conditions prevailing on both margins of this ocean. So far, no typical Afro-Brazilian chitinozoan species (e.g. Pogonochitina djalmai) have been recorded in the Saudi assemblages. Although data on the Llandovery chitinozoan assemblages from Asia are still limited, some affinities, at the specific level, between the Saudi and the Chinese material are observed.